1. Field of the Invention
The present invention relates to a multi-purpose tire for motor-vehicles.
More particularly, it pertains to the tread pattern of the tire in question, which is particularly appropriate for tires to be used on motor-vehicles envisaged for offering good performance both on a dry road and on a wet and even snow-covered road.
2. Discussion of the Related Art
It is known that in tires of the multi-purpose type the pattern and blend of the tread band must be conveniently studied so that they should be capable of meeting the different requirements specifically demanded with reference to the behavior the tire must have on the different types of road-beds for which its use is contemplated, that is not only on a dry or wet road-bed, but also on a snow-covered road-bed.
These requirement s are, on the other hand, in conflict with each other, so that hitherto-manufactured tires usually represent a compromise solution among the different requirements, and in any case they never reach the performance level achieved by a tire specifically conceived for use on a given type of road-bed, be it dry, wet or snow-covered.
Generally speaking, and leaving out of consideration the use for which it has been conceived, a good tire must have, among other things, superior features in terms of directional control and traction capability, as well as resistance to wear.
The directional control features, consisting in the tire capability to maintain the set trajectory in a precise manner, are positively affected by the presence of longitudinal grooves, whereas the traction capability features, consisting in the tire capability to transmit tangential forces both in accelerating and braking, are increased by arranging grooves oriented in a direction transverse to the running direction. The efficiency of these transverse grooves for traction capability purposes increases as the groove orientation approaches a direction parallel to the tire axis, so that usually for such grooves angles included between 45.degree. and 90.degree. are selected.
After the above preliminary remarks, it should be noted that as regards tires especially conceived for being run on a dry road-bed, the number and width of both longitudinal and transverse grooves tend to be reduced so as to improve resistance to wear, running noiselessly and smoothly of the tire.
As regards running on a wet road-bed, on the contrary, a greater groove width is desired, above all with reference to the circumferential grooves the task of which is that of discharging the water gathered on the ground-contacting area of the tire so as to avoid the occurrence of the well-known and dangerous aquaplane effect.
Finally, with reference to running on a snow-covered road-bed, a great number of cuts or narrow grooves is required, so as to conveniently hold the snow picked up from the road-bed, because--as it is well known, a snow-on-snow friction coefficient is greater than a rubber-on-snow friction coefficient.
It clearly appears from the foregoing that many difficulties are encountered when one wishes to manufacture a multi-purpose tire which simultaneously should meet all stated requirements which are in conflict with each other and specifically emerge with reference to running on a dry, wet or snow-covered road-bed.
In fact, arrangement of the circumferential wide grooves is in contrast both with the running requirements on a dry road-bed preferring grooves of reduced sizes to increase the land portion of the tread pattern so as to improve the resistance to wear and running noiselessness of same, and with the running requirements on a snow-covered road-bed, in which case wide grooves by promoting self-cleaning of the tread pattern cause a limitation in the behavioral features of the tire, in particular traction capability and roadholding.
The great number of cuts required for use on a snow-covered road-bed is in turn also in contrast with the running requirements on a dry road, because a too great number of cuts involve greater susceptibility to deformation of the land portions or blocks (which will bring about a noise increase and greater wear), as well as a reduction in the running stability and smoothness.
As a result, therefore, present multi-purpose tires have a moderately good roadholding on a wet road-bed, in any case lower than that of modern tires specifically studied for being run on a wet road-bed, a moderately good traction capability on snow, but lower than that achieved with tires specifically studied for winter climates, as well as an acceptable resistance to wear and sufficient roadholding on a dry road-bed, however without reaching the same level as the best "summer tires" available on the market.